Some 22 individuals trying to cross from Mexico into the United States illegally were killed by Border Patrol agents since 2010, The Wall Street Journal reported.

Andrea Guerrero of the Southern Border Communities Coalition, a human rights group that supports immigration reform, said, "We are very concerned about the lack of accountability, transparency, and oversight."

Immigrant advocacy groups say some of those killed were U.S. citizens.

In an undetermined number of cases, those killed had been hurling rocks at the agents. On Feb. 18, an agent who had been struck in the head by a large rock shot and killed a Mexican national, Jesus Flores-Cruz, along the border with San Diego. The agent reported using deadly force out of fear for his life, the Journal reported.

A reportby the Police Executive Research Forum, a Washington-based group that seeks to advance professionalism in policing, was critical of the way the patrol employed lethal force against unarmed civilians, rock throwers, and moving vehicles. It also said there had been a "lack of diligence" in the way these incidents were investigated, according to the Los Angeles Times.

The force, which has grown to more than 21,000 agents — up from 6,000 in 1997 — had earlier come under criticism from the Homeland Security Office Inspector General over its use of lethal force and for the agents' level of training, supervision and gear.

The Border Patrol said its agents are authorized to use deadly force only as "as a last resort" when they are in imminent danger of death or serious physical injury.

In 2012, agents came under rock assault 185 times and used firearms to respond in 22 of the cases, the Times reported.

"In a lot of cases, Border Patrol agents find themselves in an area where they don't have communications, they don't have immediate backup and often don't have the cover and concealment that urban areas provide when you are dealing with an escalation of force," Border Patrol Chief Michael Fisher told the Times.

In response to the criticism, Homeland Security Secretary Jeh Johnson said new policies on the use of deadly force would soon be unveiled. He said he was "interested in reviewing some of the more recent cases myself to ensure that we're getting this right," the Journal reported.